Thermostat

ABSTRACT

An ON, OFF switch for a load is controlled by a primary bimetallic plate or other temperature sensor, such as an hydraulic bulb. In addition to sensing the temperature of the controlled space, the primary sensor receives heat from a heater in series with the load or from the switch during the ON condition. A mechanical transmission interconnects the primary sensor and the switch. Connected in such transmission so as to be able to vary its effective length, is a secondary bimetallic plate or other temperature sensor. This secondary sensor receives from the switch some of the heat generated therein by passage of the load current. In this way the secondary sensor at least in part compensates for the effect on the switch of the heating of the primary sensor. The advantage of this arrangement is reduction of &#39;&#39;&#39;&#39;droop&#39;&#39;&#39;&#39;. There is a heater for heating the secondary sensor in the OFF condition of the switch. The combined effect of the two heaters and the two sensors is to enable a relatively high cycle rate and small &#39;&#39;&#39;&#39;room swing&#39;&#39;&#39;&#39; to be obtained at the same time as a relatively small &#39;&#39;&#39;&#39;droop&#39;&#39;&#39;&#39;.

United States Patent [1 Dalzell [111 3,813,630 [451 May 28, 1974 THERMOSTAT An ON, OFF switch for a load is controlled by a primary bimetallic plate or other temperature sensor, such as an hydraulic bulb. in addition to sensing the temperature of the controlled space, the primary sensor receives heat from a heater in series with the load or from the switch during the ON condition. A mechanical transmission interconnects the primary sensor and the switch. Connected in such transmission so as to be able to vary its effective length. is a secondary bimetallic plate or other temperature sensor. This secondary sensor receives from the switch some of the heat generated therein by passage of the load current. in this way the secondary sensor at least in part compensates for the effect on the switch of the heating of the primary sensor. The advantage of this arrangement is reduction of droop. There is a heater for heating switch. The combined effect of the two heaters and the two sensors is to enable a relatively high cycle rate 1 [57] ABSTRACT [76] Inventor: James Welland Dalzell, 624-l4th St., East, Brandon, Manitoba, Canada [22] Filed: May 21, 1973 [21] Appl. No.: 362,228

[52] US. Cl 337/360, 337/38, 337/104, 337/ 107 [51] Int. Cl. H0lh 37/12 [58] Field of Search 337/38-40, 337/77, 102, I04, 107, 95, 360, 370, 371, 377

r [56] References Cited UNITED STATES PATENTS the secondary sensor in the OFF condition of the 3:629:770 l2/l97l. Fortier 337/360 X 3 94 61972 K I' k 337 360 X l I and small room swing to be obtained at the same Primary Examiner j Miller time as arelatively small droop. Assistant Examiner-Fred E. Bell 9 Claims Drawing Figures minnow m I 3313.830

SHEU 2 BF 5 PATENTEDMYZB 1914 I 3,818,630

sum 5 or 5 This invention relates to a thermostat. While the fea tures of the present invention render it especially valuable for incorporation in a thermostat for use in controlling electrical space heating equipment and more particularly. a line voltage thermostat, the invention is applicable to any thermostat (for example a low voltage thermostat) in which the load being monitored (heating or cooling equipment) is controlled by a switch through which an electric current flows.

Since the advantages of the invention are most apparent when'applied to a line voltage thermostat the invention will be primarily explained in this context. A line voltage thermostat itself carries all the power that is supplied to an electric heating element or elements. Thermostats, and particularly line voltage thermostats, have the disadvantage commonly referred to as droop. Droop is in effect a downward movement of the-control temperature and is primarily the result of the distorting effect on the bimetallic element or other temperature sensor of the heat generated in the thermostat switch by the R loss of the current that flows through it in the ON condition. The thermostate thinks the-space under control is warmer than it really is. In a line voltage thermostat, the amount of droop is proportional to the size of the load and to the percentage of ON time. Droops as high as 40F. have been observed with resistance-type heating of 5,000 watts at 100 percent of ON time. In thermostats of the type that control a remote relay which in turn controlsthe load, the thermostat switch usually carries a current significantly less than that flowing through a line voltage thermostat, but the general construction of the thermostat will be correspondingly lighter and the PR loss can still be sufficient to produce an undesirable droop.

Various proposals have been made in the past for overcoming this difficulty, but none entirely satisfactorily.

An objectof the present invention is to provide a thermostat with improved response characteristics, especially in respect of reduction of droop.

Another object of a preferred form of the invention is to provide a single design of line voltage thermostat capable of being used in different installations, i.e. some with large loads (5,000 watts) and some with relatively light loads (500 watts), without requiring any basic changes to the themiostat.

Another problem often encountered with prior thermostats is an excessive room swing", i.e.- the range over which the temperature of the space under control varies. A typical such swing would be about F.

Such a large swing is considered undesirable, and it is accordingly one of the objects of the preferred embodiment of the present invention to provide a thermostat in which such swing can bereduced.

Generally speaking, experience with prior designs of thermostat has shown that efforts to reduce swing have resulted in an increase in droop and vice versa. Accordingly, it is a further object of the preferred form of the present invention to provide a thermostat structure in which both the droop and swing'characteristics can be improved simultaneously.

Embodiments of the invention are illustrated diagrammatically in the accompanying drawings, it being understood that these embodiments are provided only by way of example of the invention, which in its broad 2 scope is not limited to thestructural details thereof, but only to those features defined in the appended claims.

In the drawings:

FIG. 1 is an elevation view of the front of a thermostat according to the invention;

FIG. 2 is a side view as seen on the line II-ll in FIG. 1;

FIG. 3 is an end view as seen on the line III-Ill in FIG. 2;

FIG. 4 is a section on the line IV-IV in FIG. 1;

FIG. 5 is an exploded view of an adjusting mechanism forming part of the thermostat of FIGS. 1 to 4;

FIG. 6 is a sectionon the line VI-VI in FIG. 4;

FIG. 6a is an underside view of the switch assembly and associated parts, approximately as seen on the line VIa-Vla in FIG. 4, but with portions of the base omitted for clarity;

FIG. 7 is a section on the line VII-'Vll in FIG. 4;

FIG. 8 is a circuit diagram;

FIG. 8a is a fragment of FIG. 8 showing a modification thereof;

FIG. 8b isan alternative circuit diagram;

FIG. 9 is a side view partly cut away of an alternative embodiment; I

FIG. 10 is a view on the line X-X in FIG. 9; and

FIG. 11 is a view similarto FIG. 10 showing an alternative thereto.

HOUSING CONSTRUCTION (FIGS; 1 TO 7 The thermostat shown in FIGS. 1 to 7 consists of an outer housing or cover 10 and an inner housing or base 11 connected to the cover 10 and serving tosupport a switch housing 12 containing a conventional snap switch 13.

The cover 10 and base 11 are connected together by the engagement of hooks 14 projecting upwardly from the lateral edges of one end of the cover 10 to extend through correspondingly located slots 15 and engage ledges 16 formed in the base 11. At the other end of the device a raised boss 17 on the cover 10 engages the undersurface of the base 11 at 18, these parts being held together by screws (not shown). As a result of this manner of mounting, the cover 10 and base 11 are spaced from each other except at their ends, an air space 19 being formed between them.

The cover 10 contains a primary temperature sensor 20 in the form of a bimetallic plate, the main portion of which is exposed to the atmosphere through a rectangular window 21 formed in the cover 10. At one end, the plate 20 is provided with ears 22 hinged to the cover 10 by a pin 23. At its free end 24 the sensor 20 is acted upon by a calibration screw 25 of a temperature adjustment assembly 26. I

The air space 19 provides some thermal insulation between the sensor 20 and the switch 13 in which heat is generated by the load current.

In practice, of course, the thermostat is designed to be mounted in a cavity in a wall with the main portion of the sensor 20 lying in a vertical plane and suspended from the pin 23 which will be at the upper end of the device, the adjustment assembly 26 being at the lower end.

ADJUSTMENT ASSEMBLY 26 (FIG. 5)

The assembly 26 which is shown in an exploded form in FIG. 5 is essentially conventional in nature, the

Manual rotation of the setting knob 38 will cause the cam stem 27 as well as such knob to move'in or out within the bobbin 30, as controlled by the cooperation of the pin 29 and helical groove 28, such movement being conveyed by the calibration screw to the free end24 of the bimetallic sensor plate 20. The limits of rotationaltravel of the knob 38 are determined by a stop cam 35 that cooperates with a fixed pin 36 on the cover 10.

To change the range of travel of the setting knob 38, the bobbin can be brought to a new initial position by means of a calibration gear 37 that engages teeth 39 on one of the flanges 31 of thebobbin 30, the gear 37 being mounted on a member 40' supported in a cavity 41 adjacent to and communicating with thecavity 33 in the cover 10. The parts 37 and 40 are held in position by a spring 43 and can be adjusted rotationally by engaging a screwdriver in a slot 42 provided for this purpose. I

As will be evident, the rotational adjustment of the setting knob 38 and hence the position of the free end 24 of the bimetallic sensor plate 20 will represent the setting of the thermostat in terms of the desired temperature of the space under control.

MECHANICAL TRANSMISSION AND SECONDARY TEMPERATURE SENSOR (FIGS. 4, 6 and 6a) Bearing against a central location of the sensor plate 20, i.e. between its ears 23 and free end 24, so as to be influenced both by the initial setting of the knob 38 and by flexing of the sensor 20 itself due to changes in the temperature sensed, is an operating pin 50 of thermally insulating material slidably mounted in a sleeve 51 projecting from the base 11.

At its other end, the pin 50 has a head 52 that engages an outer surface of one leg 53 of a secondary temperature sensor 54 in the form of a U-shaped bimetallic element, the other leg 55 of which element bears against an operating pin 56 of the snap switch 13. Thus the secondary sensor 54 is arranged in the transmission between the primary sensor 20 and the snap switch 13, such transmission being otherwise represented by the operating pins 50 and 56.

The secondary sensor 54 embraces a heater resistor 57, the leads 58 to which are comparatively rigid wires that pass through slots 59 in the switch housing 12 (see FIG. 6a) in such'a manner as to act as pivot pins rotationally mounting the-heater 57 and sensor 54. Some slight rotation of this element is necessary to transmit mechanical motion from the first operating pin 50 to the second operating pin 56. When the heater 57 is energized to heat the sensor 54, its legs 53 and 55 move towards each other to shorten the effective length of this mechanical transmission. A resistor 57 is connected in series with the heater 57, the leads being connected to terminal strips 58 for external connections (not shown).

HEATER 60 (FIGS. 4 AND 7) A second heater resistor 60 is mounted on projections 61 extending from the base 11 so as to be located in close proximity to the end of the sensor 20 near its pivotal axis. By thus locating this heater near such axis. changes in spacing between the heater and the sensor plate due to movements of the latter, and hence changes in the heat transfer characteristics, are kept small.

CIRCUIT ARRANGEMENTS (FIGS. 8, AND 81)) FIG. 8 shows a circuit diagram of the main parts with theprimary temperature sensor 20 shown acting mechanically through the transmission (pins 50, 56) and the secondary temperature sensor 54 on the switch 13. The heater 57 is connected across the snap switch 13, while the heater 60 is in series with such switch as well as with the load L, across lines L1 and L2. The heater 57 hasa relatively high resistance (e.g. to kilohms),,while the heater 60 consists of a relatively low resistance (e.g. .002 ohms). Thus, when the switch 13 is open, i.e. in .the OFF condition, substantially all the voltage drop in the series circuit of the load L and heaters 57 and 60 is across the heater 57 and the resistor 57'. Only a small current (e.g. a few milliamps) passes, sufficient to generate the necessary heat (e.g. about 300 milliwatts) in the heater 57. Comparatively little heat is generated in the resistor 57. This resistor, which can be omitted if desired, is primarily provided to furnish a resistor that can readily be short circuited by the electrician if he is called upon to install the thermostat in a lower voltage system, e.g. a 208 volt rather than 240 volt system. A negligible amount of heat is generated at this time (i.e. in the OFF condition) in the heater 60.

On th other hand, when the switch 13 is closed, i.e. in the ON condition, the resistors 57, 57 are shortcircuited and the much larger load current passes through the heater 60 which is then effective to supply a desired amount of heat to the primary temperature sensor 20.

FIG. 8a shows a modification employing a double throw switch 13a, which system operates essentially in the same manner as FIG. 8.

In a case where a double pole switch 13b and 13c is used, (FIG. 8b), the heater 60 will remain in series with the load, while the heater 57 will be connected across the line by a further pair of contacts at 13b that are closed when the two main contacts connecting the load L to the lines are open. In this case the pin 56 acts on the switch pole 13b, which in turn is linked to the pole 130, a manual ON-OFF operating member 62.

If the thermostat is not to be connected as a line voltage thermostat, the heating element load L of these circuits is replaced by a load in the form of a relay that controls the actual heating element or other device.

OPERATION Certain of the elements of the device serve dual functions, for which reason it is believed that the easiest way to gain an appreciation of the overall function is to break the system down into three sub-systems.

(a) The compensating sub-system This sub-system consists of four elements, the temperature sensors and 54, the heater 60 and the switch 13 in which the load current generates heat.

The sensor 54 is mounted close to the switch 13 and in comparatively good thermal contact therewith. This sensor is hence heated by the switch-generated heat, while the sensor 20 remains comparatively unaffected by such heat, since it isthermally insulated from the switch.

When the switch 13 is ON the sensor 54 is heated by the switch-generated heat. At the same time the heater 60 is energised, so that the sensor 20 is heated thereby. The effect of the latter is to flex the centre of the sensor 20 towards the switch 13, i.e. to move the pin 50 towards the switch. If this motion were fully transmitted through the pin 56 it would quickly have the effect of turning the switch OFF, i.e. an anticipating effect, but the transmission length is shortened by movement together of the legs of the heated sensor 54. These two movements thus tend to cancel each other and tend to be independent of the size of the load and of the duty cycle, i.e. percentage of ON time, because the same it is impossible to achieve exactly equal compensation,

under all conditions and in all production models. In this respect, itis preferable to lean towards the side of somewhat undercompensating (i.e. making the effect on sensor 54 less than that on sensor 20) rather than towards overcompensating. Undercompensation retains some droop; overcompensation could result in negative droop and less control over the system with a decrease in the cycle rate which is generally undesirable.

thus the compensating sub-system functions primarily to compensatefor (i.e. largely eliminate the effect of) the switch-generated heat; while at the same time, insofar as the dimensions are so chosen that the compensation is made a slightundercompensation, providing some measure of anticipation. *Anticipation" is defined as turning the switch 13 OFF (or ON) before it would be turned OFF (or ON) solely by virtue of the effect on the main sensor 20 of the rise (or fall) in temperature of the space under control.

(b) The anticipating sub-system Superimposed on the function of the compensating sub-system just described, there is an anticipating subsystem that provides the main anticipating effect and consists of two elements, the sensor 54 and its heater 57.

The heater 57 is energised in the OFF condition of the switch, at which time the heater 60 is inoperative and there is no switch-generated heat. Hence the heater 57 is now essentially the only source of heat for the sensor 54, which is thus flexed when the heater 57 is energised to its heated condition, the effect of which is to shorten the mechanical linkage, i.e. to allow the pin 56 to move outwards and turn ON the switch 13 before the fall in temperature of the controlled space has become sufficient to cause the main sensor 20 to achieve the same effect; in other words anticipation. Conversely, in the ON cycle the lack of heat from the heater 57 will havethe effect of allowingthe sensor 54 to cool, thus spreading its legs and tending to move the switch 13 to the OFF condition, which is also an anticipating action.

It must be appreciated that the compensating and anticipating sub-systems are operating simultaneously and with certain parts in common (primarily the sensor 54). Thus the actual total operation that takes place is an amalgam of the sub-system operations described above, neither'of which latter therefore represents the full truth, but which have been separately described as a convenient way of breaking down the complex function of the device for the purposes of understanding. (c) The space sensing sub-system Still further superimposed on the combined operation of the two sub-systems described above, there is a third sub-system comprising the main sensor 20, the switch 13 that it operates, the heater 60 which in this sub-system functions simply as an anticipator, and of course the mechanical linkage between the sensor 20 and the switch 13 (the pins 50 and 56 and the sensor 54).

Basically, the sensor 20 functions in this sub-system in the conventional manner of a thermostat, i.e. to sense. the temperature of the controlled space and move the switch accordingly. However, this action is complicated by the anticipating effect of the heater 60. Since the heater 60 is in series with the load, the amount of heat it generates is proportional to the heat being generated by the load. Under maximum conditons, i.e. a large load and a heavy duty cycle, say 90 percent of ON time, the relatively large amount of heat imparted to the sensor 20 by the heater 60 causes the temperature of such sensor slowly (it has a comparatively long time constant) to reach a value significantly above that of the controlled space. Under these conditions the effect on the sensor 20 of a change in the temperature of such space is to change the rate of heat loss to the space from the sensor 20. It has been found that, as a result of this fact, under heavy load conditions the sensor 20 tends to react more quickly to space temperature changes than it does under light load condition when the total heat output from the heater is low and the sensor 20 merely follows the space temperature.

Yet another way of looking at the effect in the space sensing sub-system of the sensor 54 and its associated heater 57 (which together form a link in the mechanical linkage) is that these elements provide a means for changing the rate of operation of the device. Normally the time constant of the elements 54, 57 will be short, compared with that of the elements 20, 60, but such time constant can be varied by variations of mass, thermal capacity and thermal conductivity. This provision of a link in the mechanical linkage between the main sensor and the switch, that can have its time constant chosen to optimise conditions, is a valuable tool in the hands of the designer and hence an asset of the device. Generally speaking, a decrease in the time constant of elements 54, 57 causes an increase in the cycle rate but an increase in hunting. A relatively high cycle rate, between eight and 12 full cycles (one full cycle comprising one ON and one OFF cycle) per hour is normally desirable, except in an electric heating system of the cable type where a slower rate is often acceptable.

ln convection heating systems a high cycle rate leads to a small room swing". As mentioned above a small room swing is one of the desirable features of the present device. l

On the other hand in cable heating systems, room swing is usually less of a problem, but it is desirable to minimise hunting. Hunting is defined as variation of the length of individual ON and OFF cycles from the average length of such cycles. As indicated above a small time constant for the elements 54, 57 can increase hunting. Hence, with a thermostat destined for use with a cable heating system, the present device has the valuable versatility that by simply increasing such time constant, hunting can be reduced.

ALTERNATIVE CONSTRUCTIONS (FIGS. 9 TO 11) An alternative construction is illustrated in FIGS. 9

and 10, in which the primary temperature sensor has been changed from a bimetallic plateto an assembly of an hydraulic bulb 70 connected by a conduit 71 to a bellows 72. An adjustment mechanism 73 operated by the setting knob 38 has a screw portion 74 that determines the position of a sleeve 75 mounted on a plate 76 pivotable about a pin 77 which also extends through ears in a fixed plate 78 secured to the base 11. One end of the bellows 72 acts against the plate 76 at 79 and the other end of the bellows 72 acts at 80 against one end of a plate 81 that is also pivoted about the pin 77 and acts at 82 (on the other side of its pivotal axis) against the operating pin 50, the parts being held firmly in position by a coil spring 83 extending coaxially around the pin 50 between the plate 81 and the base 11. The remainder of the structure, including the secondary temperature sensor 54 and heater 57, is unchanged, except that the heater 60 is now placed so as to heat the tube 70.

In a like manner, the secondary temperature sensor could take the form of an hydraulic arrangement with bellows replacing the bimetallic plate 54 and a suitably located tube arranged to receive heat both from the heater 57 and from the heat generated in the switch.

The use of hydraulic rather than bimetallic sensors facilitates sensing at locations remote from the switch assembly as demonstrated in FIG. 11 which shows a further alternative. In FIG. 11 the bulb 70 has been mounted remotely and the heater 60 has been omitted. To duplicate the function of the heater 60, a subsidiary hydraulic bulb 70 has been mounted on the switch 13 to receive the switch-generated heat, the bulb 70' being connected in parallel with the bulb 70 to the bellows 72. Typically the volume ratios of 70 to 70' to 72 might be approximately 40 4 1.

Yet a further alternative construction (not illustrated) retains the heater 60 so located as to heat the subsidiary bulb 70'. The bulb 70' can then be arranged to receive heat either only from the heater 60 or from both such heater and the switch. As in the main embodiment described above, it will be apparent that the various dimensions and conditions can be so chosen as to achieve the-desired degree of compensation in the compensation sub-system and the desired degree of anticipation in the space sensing sub-system.

I claim:

1. A thermostat for controlling the temperature of a space, comprising a. a switch having an ON and an OFF condition,

b. means for connecting said switch in the ON condition in series with a load,

c. primary temperature sensor means for sensing the temperature of said space,

d. a transmission connected between said primary sensor means and said switch for actuating the latter upon movement of the former.

e. means for applying heat proportional to load current through the switch to said primary sensor means during the ON condition of said switch,

f. secondary temperature sensor means located adjacent the switch to receive heat generated in the switch by load current during the ON condition,

g. said secondary sensor means being connected in said transmission to vary the effective length thereof in accordance with the temperature sensed by said secondary sensor means whereby at least in part to compensate for the effect on the switch of said primary sensor means,

h. a heater located for heating said secondary sensor means,

i. and means for energising and heater during the OFF condition.

2. A thermostat according to claim 1, wherein said means for applying heat to said primary sensor means comprises a further heater located adjacent said primary sensor means, and means for connecting said further heater in series with the load.

3. A thermostat according to claim 2, wherein said primary sensor means comprises a bimetallic plate.

4. A thermostat according to claim 1, wherein said secondary sensor means comprises a U-shaped bimetallic plate having a pair of legs forming a cavity between them, said heater being located in said cavity.

5. A thermostat according to claim 4, wherein said transmission comprises a first pin extending from the primary sensor means to one leg of the secondary sensor means and a second pin extending from the other leg of the secondary sensor means to the switch.

6. A thermostat according to claim 1, wherein said primary sensor means comprises an assembly of an hydraulic bulb and bellows.

7. A thermostat according to claim 6, wherein said means for applying heat to said primary sensor means comprises a further heater located adjacent said bulb, and means for connectingsaid further heater in series with the load.

8. A thermostat according to claim 6, wherein said means for applying heat to said primary sensor means comprises a subsidiary hydraulic bulb connected to said bellows, a further heater located adjacent said subsidiary bulb, and means for connecting said further heater in series with the load.

9. A thermostat according to claim 6, wherein said means for applying heat to said primary sensor means comprises a subsidiary hydraulic bulb connected to said bellows, said subsidiary bulb being located adjacent the switch to receive heat generated in the switch by the load current during the ON condition. 

1. A thermostat for controlling the temperature of a space, comprising a. a switch having an ON and an OFF condition, b. means for connecting said switch in the ON condition in series with a load, c. primary temperature sensor means for sensing the temperature of said space, d. a transmission connected between said primary sensor means and said switch for actuating the latter upon movement of the former. e. means for applying heat proportional to load current through the switch to said primary sensor means during the ON condition of said switch, f. secondary temperature sensor means located adjacent the switch to receive heat generated in the switch by load current during the ON condition, g. said secondary sensor means being connected in said transmission to vary the effective length thereof in accordance with the temperature sensed by said secondary sensor means whereby at least in part to compensate for the effect on the switch of said primary sensor means, h. a heater located for heating said secondary sensor means, i. and means for energising and heater during the OFF condition.
 2. A thermostat according to claim 1, wherein said means for applying heat to said primary sensor means comprises a further heater located adjacent said primary sensor means, and means for connecting said further heater in series with the load.
 3. A thermostat according to claim 2, wherein said primary sensor means comprises a bimetallic plate.
 4. A thermostat according to claim 1, wherein said secondary sensor means comprises a U-shaped bimetallic plate having a pair of legs forming a cavity between them, said heater being located in said cavity.
 5. A thermostat according to claim 4, wherein said transmission comprises a first pin extending from the primary sensor means to one leg of the secondary sensor means and a second pin extending from the other leg of the secondary sensor means to the switch.
 6. A thermostat according to claim 1, wherein said primary sensor means comprises an assembly of an hydraulic bulb and bellows.
 7. A thermostat according to claim 6, wherein said means for applying heat to said primary sensor means comprises a further heater located adjacent said bulb, and means for connecting said further heater in series with the load.
 8. A thermostat according to claim 6, wherein said means for applying heat to said primary sensor means comprises a subsidiary hydraulic bulb connected to said bellows, a further heater located adjacent said subsidiary bulb, and means for connecting said further heater in series with the load.
 9. A thermostat according to claim 6, wherein said means for applying heat to said primary sensor means comprises a subsidiary hydraulic bulb connected to said bellows, said subsidiary bulb being located adjacent the switch to receive heat generated in the switch by the load current during the ON condition. 